Program Director/Principal Investigator (Last, First, Middle): Oldstone, Michael - 2 R01 AI055540-06 REVISED ABSTRACT SECTION Viral hemorrhagic fevers (VHF) caused by the arenaviruses Lassa virus (LASV), Machupo (MACV), Guanarito (GTOV), and Junin virus (JUNV) are devastating human diseases with fatality rates of 15 to 35%. There is limited antiviral treatment available. Using high-throughput screening of synthetic combinatorial small molecule libraries coupled with pseudotyped virion particles bearing the glycoproteins (GPs) of these highly pathogenic arenaviruses, we identified viral inhibitors. Our candidate small molecule inhibitors are highly active against both Old World and New World hemorrhagic arenaviruses. Initial studies revealed they efficiently block pH-dependent fusion by the arenavirus GPs (IC50 of 200-350 nM) and blocked infection by these human pathogenic viruses in cultured cells when tested in BSL-4 at the CDC. Our current proposal plans to optimize the most potent lead compounds by iterations of secondary library synthesis and screening. Optimized compounds will be tested for their anti-viral potential in cell culture and inhibitory potential against LASV assessed in a newly developed small animal model. The detailed mechanism of action of our lead candidates will be investigated and the development of drug resistant viral variants addressed. We anticipate making a significant contribution to drug development to treat these BSL-4 agents.